Highly aromatic waste containing polycyclic aromatic hydrocarbons (PAHs) are generated by several industries such as gas plants, blast furnaces coke oven condensation, wood preservation plants, petrochemical industry sites, and other industries. PAHs are of great environmental concern because of their mutagenic and/or carcinogenic potential and their nature to persist in the environment for very longer durations. Various methods have been utilized for treatment/remediation/disposal of highly aromatic waste sludge. These methods generally include permanent removal of the contaminated soil to a secure landfill, incineration, indirect thermal treatment, aeration, venting, air sparging and bioremediation.
Of these, removal of contaminated soil to landfills is no longer an attractive alternative on account of the high excavation, transportation, disposal costs, and a high potential for residual liability. Further, the contaminant has to be dumped into specially constructed pits with leachate collection system in the landfill. Due to space constraints and expensive construction of such pits, this option is not practical. Further, this method is not a permanent solution.
Incineration and indirect thermal treatment can be achieved either on-site or off-site, but both the processes involve excavation, handling and treatment of whole contaminated soil as well as significant amounts of soil adjacent to the contaminated soil. The soil must then either be transported to the treatment facility or else the treatment apparatus must be installed on-site. In either case, these methods generally involve enormous transportation, handling cost and require large amounts of energy to combust or volatilize the contaminants. Other elaborate and expensive techniques, which have been utilized, involve excavation and treatment of the contaminated soil using multi-step unit operations for separating and recovering the soil from the contaminants. The processes mentioned above are prohibitively expensive.
Bioremediation is generally considered to be a promising approach to remediate contaminated soils, which utilizes the ability of certain microbes to transform harmful substances in to nontoxic compounds. Microorganisms, like all living organisms, need nutrients (such as nitrogen, phosphate, and trace metals), carbon and energy to survive. Bioremediation may be affected under aerobic and anaerobic conditions. Main requirements for effective bioremediation are: a biodegradable organic substrate, an appropriate active microbial community (consortium) and bioavailability of contaminants. Sometimes bioremediation requires further biostimulation with nutrients or some specific analogue substrate. Bioremediation is a cost effective, less energy intensive and eco-friendly method for disposal of hydrocarbon sludge. However, biodegradation of highly aromatic waste is a difficult task on account of their polynuclear chemical structure, low aqueous solubility and high toxicity. Various methods have been tried in the prior arts for treatment of the aromatic wastes.
U.S. Pat. No. 5,055,196 to Darian et al., discloses a process for treating soil or sludge to remove contaminants. More particularly, this method relates to a process in which inorganic contaminants, such as metal or metal salts, or organic contaminants, such as polychlorinated biphenyls (PCBs), are removed from water-wet soil and sludge by contacting the contaminated water-wet mixture with a solvent containing a comminuting surfactant.
U.S. Pat. No. 5,427,944 to Lee et al. teaches a process for biodegradation of polycyclic aromatic hydrocarbon contaminants using a mixed bacteria culture of Achromobacter sp. and Mycobacterium sp and nutrient. The mixed bacteria culture was utilized for in situ or ex situ bioremediation of contaminated soil, or in any of various conventional bioreactors to treat contaminated liquids such as landfill leachates, groundwater or industrial effluents.
U.S. Pat. No. 6,251,657 to Hunter et al. discloses an apparatus and method for anaerobic biodegradation, bioremediation or bioprocessing of hydrocarbon dissolved in aqueous matrix, such as wastewater, ground water, or slurry and dissolved alkanes, alkenes, aromatic hydrocarbons and/or halogenated hydrocarbons that are metabolized or co-metabolized by denitrifying bacteria.
U.S. Pat. No. 6,381,899 to McDole, describes a method for converting well drilling cuttings that contain petroleum hydrocarbons to an environmentally friendly humus-like product wherein salt, if present, is washed there from, the washed cuttings mixed with a feedstock having a carbon and nitrogen content sufficient to encourage a biopile composting reaction, forming an environmentally acceptable plant growth enhancing humus-like product by continuing the biopile composting reaction until (1) the biopile is reduced in weight and/or volume to a total biopile weight and/or volume that approaches the original weight and/or volume of the drill cuttings before the feedstock was added thereto, and (2) the total petroleum hydrocarbon content of the biopile is reduced to an environmentally friendly level for spreading on the earth's surface.
U.S. Pat. No. 6,503,746 to Daane et al. relates to a method of isolating bacterial strains of Paenibacillus validus that degrade polyaromatic hydrocarbons and further use these strains for bioremediation.
U.S. Pat. No. 7,118,906 to Raghukumar et al. describes a process for removal of polycyclic hydrocarbons phenanthrene and chrysene from wastewater and other contaminated sites by using white-rot lignin modifying fungus strain Flavodon flavus NRRL 30302.
The US 20030100098 to Haggblom et al. discloses a bioremediation method of polycyclic aromatic hydrocarbon employing a PAH degrading bacterial stain P. naphthalenovorans of the Paenibacillus genus.
The US 20050221468 discloses a process and a system for in situ remediation of contaminated soil by the distribution of treated sewage effluent into the soil thereby promoting the number and growth of naturally occurring microorganisms for the remediation process.
The US 20060275887 to Miller et al. teaches a contaminant-degrading mycobacterium composition for remediation of contaminated soil having selected contaminants such as PAHs, polychlorinated phenols (PCPs), methyl tertiary butyl ethers (MTBEs) and the like. Such a composition includes a seed for plant capable of growing in the presence of contaminant and contaminants degrading mycobacteria on the seed, the mycobacterium being capable of degrading the selected contaminant.
The conventional bioremediation techniques utilizing indigenous microorganisms alone or in combination with naturally occurring exogenous microorganisms are not always effective for bioremediation of polycyclic aromatic hydrocarbons. Polycyclic aromatic hydrocarbons (PAHs) are strongly adsorbed with organic material present in the soil, which limits their biodegradation in land farming approach and often-high levels of residual contaminants are left. Hence, the aromatic waste cannot be bioremediated using normal hydrocarbon degrading bacteria and by land farming techniques.
Therefore, there exists a need to develop a method which can remove highly aromatic compounds especially PAHs in an eco-friendly, economical and intensive manner.